Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q17/00—Devices for absorbing waves radiated from an antenna; Combinations of such devices with active antenna elements or systems
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
  • G01R33/34—Constructional details, e.g. resonators, specially adapted to MR
  • G01R33/345—Constructional details, e.g. resonators, specially adapted to MR of waveguide type
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
  • G01R33/34—Constructional details, e.g. resonators, specially adapted to MR
  • G01R33/34007—Manufacture of RF coils, e.g. using printed circuit board technology; additional hardware for providing mechanical support to the RF coil assembly or to part thereof, e.g. a support for moving the coil assembly relative to the remainder of the MR system
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T29/00—Metal working
  • Y10T29/49—Method of mechanical manufacture
  • Y10T29/49002—Electrical device making
  • Y10T29/4902—Electromagnet, transformer or inductor
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T29/00—Metal working
  • Y10T29/49—Method of mechanical manufacture
  • Y10T29/49002—Electrical device making
  • Y10T29/4902—Electromagnet, transformer or inductor
  • Y10T29/49073—Electromagnet, transformer or inductor by assembling coil and core

Definitions

• This invention pertains generally to magnetic resonance imaging (MRI) and more specifically to surface and volume coils for MRI imaging and spectroscopy procedures.
• MRI magnetic resonance imaging
• MRI and volume coils are used in MRI imaging or spectroscopy procedures in order to obtain more accurate or detailed images of tissue under investigation.
• a MRI coil performs accurate imaging or spectroscopy across a wide range of resonant frequencies, is easy to use, and is affordable.
• the operating volume inside the main magnet of many MRI systems is relatively small, often just large enough for a patient's head or body. As a result, there is typically little space available for a coil in addition to the patient. Accordingly, it is advantageous if a surface or volume coil itself occupies as little space as possible.
• MRI coils according the present invention are easy to manufacture with relatively low cost components, and compact in design.
• the coil's distributed element design provides for operation at relatively high quality factors and frequencies and in high field (4 Tesla or more) environments.
• microstrip coils according to the present invention exhibit relatively low radiation losses and require no RF shielding.
• the coils may be of compact size while having high operating frequencies for high field MR studies, thus saving space in the MRI machine.
• the methods and apparatus of the present invention are not just good for high frequency MR studies, but also good for low frequency cases.
• Figure 1 illustrates a method according to one example embodiment of the invention.
• Figures 2-14 illustrate various example embodiments of the apparatus of the invention.
• a target is positioned within the field of a main magnetic field of a magnet resonance imaging (MRI) system, at least one coil is positioned proximate the target wherein the coil is constructed using at least one microstrip transmission line, and the main magnet and the MTL coil are used to obtain MRI images from the target.
• the microstrip transmission line (MTL) coil it is operated as a receiver (pickup coil) or a transmitter (excitation coil) or both during an imaging procedure.
• the term "MTL coil” generally refers to any coil formed using a microstrip transmission line.
• the microstrip transmission line is formed of a strip conductor, a ground plane and a dielectric material that may be air, a vacuum, low loss dielectric sheets such as Teflon or Duroid, or liquid Helium or liquid Nitrogen.
• the strip conductor or ground plane are, in one embodiment, formed in whole or in part from a non-magnetic conductive material such as copper or silver.
• the ground planes for multiple strip conductors are arranged in one single piece foil so as to reduce radiation loss.
• the MTL coil is a volume MTL coil having a plurality of microstrip transmission lines.
• the volume MTL coil is detuned using PIN diodes.
• the MTL coil includes bisected ground planes and the PIN diodes are positioned in the gap of the bisected ground planes.
• a MTL coil is tuned by varying capacitive termination of the MTL coil wherein, for example but not by way of limitation, the MTL coil is tuned by varying capacitive termination on each end of the MTL coil.
• the microstrip transmission line is arranged in a rectangular or circular configuration, or, in the alternative, in an S shape, h one advantageous embodiment, the MTL coil is constructed using at least two turns to improve the homogeneity of the magnetic field characteristics.
• one or more lumped elements are connected to the transmission line and operated so as to match the impedance of the line.
• an MTL coil is operated in a resonant mode by bisection of the ground plane and tuning of the resonance by adjusting displacement of the ground planes.
• at least two of the MTL coils are operated in a quadrature mode.
• a coil is arranged so as to operate as a ladder MTL coil.
• at least two MTL coils are arranged and operated as a half volume MTL coil.
• an inverted imaging MTL coil is formed wherein the dielectric material is positioned in a plane on the side of the strip conductor plane in the direction of the field, and wherein coupling is capacitive.
• the MTL coil is driven using a capacitive impedance matching network.
• the dielectric constant Er is adjusted to change the resonant frequency of the MTL coil.
• the coil dielectric substrate is flexible, and the MTL coil is formed and used in more than one configuration allowing a single coil to be adapted to multiple purposes.
• the substrate is formed of thin layers of Teflon or other dielectric material allowing the substrate to be bent or twisted. Apparatus Embodiments Referring first to Figures 2 A and 2B, there is illustrated in diagrammatic form an example embodiment of a microstrip transmission line (MTL) 20 having a strip conductor 21 with a width W and ground plane 22, on either side of a dielectric substrate 23 having a height H and dielectric coefficient Er.
• MTL microstrip transmission line
• Magnetic field lines H are shown surrounding strip conductor 21 and emanating outward in the Y direction orthogonal to ground plane 22 and along the length of strip conductor 21. As illustrated, the field is contained in whole or in part on one side of the strip conductor 21 by the ground plane 22, and extends outwardly beyond the plane of the strip conductor in a direction extending away from the ground plane.
• a single turn MRI imaging or spectroscopy MTL coil 23 constructed using at least one microstrip transmission line.
• the microstrip transmission line coil is formed of a strip conductor 24, a ground plane 25 and a substrate 26 made of a dielectric material that may be air, a vacuum, a single or multilayer low loss dielectric sheets such as Teflon or Duroid materials, or liquid Helium or Nitrogen.
• such coil is 9 cm X 9 cm, has a substrate 26 that is 5-7 mm, uses copper foil 36 microns in thickness (for example an adhesive-backed copper tape such as is available from 3M Corporation of St.
• the MRI signal intensity is proportional to H when H ⁇ 5 mm and reaches a maximum when H 5 mm.
• the strip conductor or ground plane are, in one embodiment, formed in whole or in part from a non-conductive material such as copper or silver.
• the strip conductor and ground plane in this embodiment and others described below, is comiected to a source of electrical excitation or RF detection circuitry, for example through a coax or other connector (not shown).
• the corners may be chamfered to reduce the radiation loss and improve B 1 distribution.
• a two turn coil is illustrated.
• a single ground plane 32 is shared by the strip conductors 34, wherein the ground planes are formed for example with a single sheet of foil so as to reduce radiation loss.
• Figures 3C and 3D illustrate additional embodiments 35 and 36 wherein embodiment 35 has one turn and embodiment 36 has two turns 102 to improve the homogeneity of the magnetic field characteristics.
• the coils may assume an "S" shape, as may be advantageously used for example in a volume coil design, or any other arbitrary shape.
• one or more elements 27 and 31, respectively, are connected to the transmission line so as to match the impedance of the line.
• the MTL coil 42 is a half- volume coil having a plurality of microstrip transmission lines each having a ground plane 46 and strip conductor 48.
• the MTL coil 50 includes a bisected ground plane 52. In this configuration, tuning of the resonance frequency is accomplished by adjusting displacement 54 of at least one of the ground planes.
• PIN diodes 64 are positioned in the gap 66 of the bisected ground planes 62, and used to detune the coil.
• a MTL coil 70 is tuned by varying capacitive termination elements 72 on one end of the coil.
• Figure 7B illustrates a hypothetical plot of magnetic field profile vs. capacitive termination value for a range of capacitances. As illustrated, increasing capacitive termination raises the magnetic field profile at the end of the coil at which the termination is applied.
• Figures 7C and 7D illustrate an example embodiment and field profile for tuning a MTL coil 74 by varying capacitive termination on each end 75 of the coil. Further, fine tuning can also be accomplished by slightly changing the length of the strip conductor.
• At least two of the MTL coils 80 are arranged to be operated in a quadrature mode.
• the equivalent electrical circuit for MTL coil 80 is illustrated in Figure 8B.
• Z0 is the characteristic impedance of each microstrip element.
• jX, jXl , jX2, jXl 5, X, XI , X2, XI 5 are positive real numbers.
• L denotes the length of the volume coil 80.
• an MTL coil 90 is formed as arranged and operated as a ladder MTL coil.
• a volume coil 100 is provided.
• Coil 100 includes ground planes 102 on the outside of a cylinder of dielectric material (for example Acrylic) having a diameter of 260 mm, a length of 210 mm, and amaterial thickness 104 of 6.35 mm.
• Strip conductors 106 are placed on the inside of the coil 100 running parallel to the axis.
• Coaxial connectors 108 are provided to connect the ground planes and strip conductors to a source of electrical excitation or RF detectors, as is conventionally done in use of a MRI volume or surface coil.
• the high permittivity of the human head results in higher signal intensity in the central region of the image. This higher intensity can be taken into account in the design of a large volume coil at high fields.
• an inhomogeneous Bl distribution in the transaxial plane in free space is intentionally designed to compensate for the dielectric resonance effect in the human head.
• the resonant frequency can be modified by choosing appropriate dielectric substrate with different relative dielectric constant.
• doubly tuned frequency operation can be easily achieved by making two different resonant frequencies for the microstrip elements in the volume coil, alternatively. Namely, one set of microstrip resonant elements with even numbers can be set to one resonance frequency while another set of microstrip resonant elements with odd numbers set to a different resonance frequency. Multiple tuned RF coils also can be designed using the same approach. Each resonance can be quadraturely driven with an appropriate quadrature hybrid. In still another example embodiment shown in Figure 11, an inverted MTL coil 110 is illustrated, wherein is coupling is capacitive adjacent microstrip elements to provide lower resonant frequency operation.
• the MTL coil 130 substrate dielectric is formed of one or more relatively thin flexible layers 132 so that the coil may be bent or twisted or otherwise formed. Such layers may be formed of Teflon, for example. According to this embodiment, the coil 130 may be bent or formed into a first configuration, and thereafter formed into a second or third or more different configurations, wherein the coil may be used in more than one configuration and thus have a multipurpose nature.
• FIG. 14 there is illustrated a photograph of yet one more example embodiment of a volume coil 140 according to the present invention.
• the MTL coil is formed as a dome-shaped coil which offers an increased filling factor and a great sensitivity and homogeneity in the top area of the human head.
• the dome-shaped coil can be constructed for higher field applications.
• the unbalanced circuit of the microstrip coil provides that there is no need to use the balun circuit commonly used in surface coils and balanced volume coils to stabilize the coil's resonance and diminish the so-called 'cable resonance'.
• MTL coils of the present invention are also characterized by a high Q factor, no RF shielding, small physical coil size, lower cost and easy fabrication. These MTL coils have the advantageous property of good performance while occupying a relatively small space, thus allowing MTL coils to be used inside restricted areas more easily than some other prior art coils.
• the MTL coils of the present invention can be readily formed in a wide variety of coil configurations, and used in a wide variety of ways. Further, while the MTL coils of the present invention work well at high field strengths and frequencies, they also work at low frequencies and in low field strengths as well.

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• Physics & Mathematics (AREA)
• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Magnetic Resonance Imaging Apparatus (AREA)

Applications Claiming Priority (3)

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• 2001
  • 2001-10-09 EP EP01977633A patent/EP1344076A1/de not_active Withdrawn
  • 2001-10-09 JP JP2002534856A patent/JP2004511278A/ja not_active Withdrawn
  • 2001-10-09 WO PCT/US2001/031520 patent/WO2002031522A1/en not_active Ceased
  • 2001-10-09 AU AU2001296738A patent/AU2001296738A1/en not_active Abandoned
  • 2001-10-09 US US09/974,184 patent/US7023209B2/en not_active Expired - Lifetime
• 2005
  • 2005-04-26 JP JP2005128758A patent/JP2005270674A/ja active Pending
  • 2005-09-12 US US11/224,436 patent/US20060006865A1/en not_active Abandoned
• 2006
  • 2006-05-17 US US11/436,197 patent/US20060277749A1/en not_active Abandoned

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